1. Field of the Invention
This invention relates to a double superheterodyne tuner for use in a television receiver set or an FM stereophonic receiver set, and more particularly to a double superheterodyne tuner of up-conversion type using a microstrip circuit.
2. Description of the Prior Art
Tuners may be classified into two types; single superheterodyne tuners and double superheterodyne tuners. In the past, single superheterodyne tuners have been widely used.
With the recent development of UHF circuit technology, however, the so-called up-conversion type superheterodyne reception system, in which the frequency of the first IF signal is always chosen to be higher than that of the received signal, has been increasingly adopted. This tendency is due to the merits; an excellent characteristic for rejection of interference (e.g. image and IF interference); a capability of covering a wide frequency range easily without change-over of bands; and feasibility of adjustment for every channel of a TV tuner.
In such an up-conversion type superheterodyne reception system, since the frequency of the first IF signal is rather higher than that of the received signal, the image interference with respect to the first mixer is almost negligible and only the image interference with respect to the second mixer has to be taken into consideration. The degree of the appreciable interference depends largely on the filtering characteristics of the bandpass filters used. In order to improve the image ratio, therefore, the bandpass filters to be used must be so designed as to provide a sufficient attenuation outside a desired range of frequencies.
Moreover, in the UHF region, even bandpass filter having an ideal attenuation characteristic will be useless if there is even a very slight electromagnetic coupling between the input and output sides of the filter. Namely, the choice of filter characteristic is not all that is required, but the electromagnetic coupling between the input and output sides of the filter must be eliminated to a satisfactory extent. In the tuner described above, therefore, the bandpass filter should be separated in the sense of high frequency interference from other circuits and securely grounded.
However, the conventional tuner structure has a drawback that it requires numerous shield plates to provide secure high-frequency separations and also a drawback that the size of the tuner is rather large and cannot be reduced.
Further, since the contact surface between the substrates of circuit elements and the chassis is rather large, it is difficult to maintain a uniform grounded condition. This leads to an additional drawback that a desired characteristic cannot be obtained.